Along with the progress in the electronic technologies of recent years, small-sized portable electronic devices such as camera-integrated VTRs, mobile phones, and laptop computers have been developed, and there is a strong demand for the development of small-sized and lightweight secondary batteries with high energy densities as portable power supplies.
As one type of secondary batteries that comply with such a demand, there are expectations on non-aqueous electrolyte secondary batteries that use light metals such as lithium, sodium and aluminum, which can generate high voltage in theory and have high energy densities, as negative active materials. Among them, active research and development is being conducted on lithium ion secondary batteries in which charging and discharging of lithium ions is carried out by means of a non-aqueous electrolyte liquid, as secondary batteries that are capable of realizing higher output and higher energy density when compared with nickel-cadmium batteries or lead-acid batteries, which are aqueous solution-based electrolyte secondary batteries.
Since these lithium ion secondary batteries have large internal energy, they are required of high safety at the time of abnormalities such as an internal short circuit or an external short circuit. Thus, for this purpose of safety measures, polyolefin-based microporous membranes have been put to use. This is because these microporous membranes are thought to have a function of closing pores at the time of abnormal heat generation, and thereby blocking the electricity flow (shutdown function). However, a situation may be considered in which abnormal heat generation is not halted even though such safety measures are being taken, the polyolefin-based microporous membrane undergoes contraction or melting, so that electrodes are brought into contact and short-circuited, causing a fire.
Suggested as separators which can be expected to have an ability to prevent a short circuit caused by contraction or melting as in the case of such a polyolefin-based microporous membrane, are those produced by providing a ceramic film or the like on a support (Patent Literatures 1 and 2).